This invention relates to metal refining processes in which a refining gas is blown into a bath of molten metal from both above and below to enhance the scrap melting capability of the process by combustion of gases emerging from the bath, and particularly to obtain such enhanced scrap melting capability without decreasing the life of refractories lining the vessel in which the process is carried out, especially as it relates to a Q-BOP or other bottom-blown steelmaking process.
The oxygen converter steelmaking processes permit production of steel in only a fraction of the time required in the formerly predominant open hearth process. A substantially higher proportion of molten hot metal is required in the charge of the oxygen converter processes in order to provide the heat necessary for refining. As a result of the increased use of molten hot metal, these oxygen processes suffer from the disadvantage that where a cheap supply of cold scrap is readily available, the most economical proportion of charge materials may not be used. Thus, it is important under some market conditions to increase the ability of the converter processes to utilize higher percentages of cold metal scrap in the charge.
Where increased scrap melting capability is beneficial, it has been proposed that the scrap be preheated either within the converter itself or in a separate furnace prior to charging into the converter. Both capital expenditures and operating costs would be increased if either of these procedures were to be used. It has been proposed that fuels may be burned above the bath during refining in order to provide additional heat and increase the scrap melting capability of the oxygen converter processes, particularly the bottom-blown processes. Since carbon monoxide is evolved from the bath during refining, it serves as a readily available source of fuel which may be utilized for this purpose. Thus, a secondary source of oxygen provided above the bath will cause combustion of the emerging CO generating additional heat for asbsorption by the bath.
U.S. Pat. No. 3,895,784, Kolb et al shows a Q-BOP converter having generally horizontal or slightly downwardly tilted side tuyeres for this purpose. The tuyeres are mounted at locations slightly above the bath level. In addition, tuyeres located at an upper level of the vessel are shown for the purpose of combusting CO in order to burn away solid deposits of metal commonly known as nose skulls. U.S. Pat. No. 3,960,546 also shows similar upper and lower tuyere configurations. A significant disadvantage to the use of side tuyeres is the adverse effect such use has on refractory life due to the high temperatures produced above the bath.
To overcome the problem of lower refractory life, changes in vessel dimensions have been proposed to increase the rate of absorption of heat by the molten bath. U.S. Pat. No. 4,201,572, Slamar indicates that a significant increase in the surface-to-volume ratio of the bath over that normally provided in basic oxygen furnace designs, would provide the desired rate of heat absorption. However, it would obviously be expensive to make changes in vessel dimensions. Moreover, this does not provide a solution to the problem for vessels presently used. U.S. Pat. No. 4,047,936, Chang shows tuyeres or lances located further above the bath level and tilted at greater angles so as to direct the secondary oxygen downwardly toward the bath. This reference also suggests that a vertical water-cooled lance extending downwardly into the vessel may alternatively be used to inject oxygen into the area above the bath. U.S. Pat. No. 4,195,985, Brotzmann indicates that the degree of heat absorption into the bath using such downwardly angled tuyeres or a vertical lance may be substantially increased by proper adjustment of the proportion of top and bottom oxygen flow and other variables. In this case it is suggested that high temperatures in the upper part of the vessel are not present so that a decrease in refractory life is not experienced. But the lance and tuyere configurations disclosed in Chang and the blowing conditions disclosed in Brotzmann do not prevent the formation of nose skulls. The present invention is directed to increasing the scrap melting capability of bottom-blown metal refining processes without decreasing refractory life and simultaneously preventing the formation of skulls in the cone and mouth of the vessel. U.S. Pat. No. 3,960,546, Brotzmann suggests that a single lance inserted through the vessel mouth may be effective for preventing formation of nose skulls but contains no suggestion that such a lance may be located above the vessel mouth rather than within it, nor that a lance in this area may be used to transfer heat to the bath increasing scrap melting capability of the process as well as preventing formation of nose skulls. It has been found quite unexpectedly that a secondary oxygen lance located outside the vessel will accomplish all of these objectives. It is also believed that greater scrap melting capability is achieved when the lance is located above the vessel as compared to within the vessel. To applicant's knowledge, no one has ever used a lance located outside the vessel for increasing the scrap melting capability of an oxygen-blown steelmaking process.